1. Technical Field
The present invention relates to an epitaxial substrate that has a multi-layer structure composed of a group III nitride semiconductor, and more particularly, to a multi-layer structured epitaxial substrate for electronic devices and a method of manufacturing the same.
2. Description of Related Art
Nitride semiconductors having high breakdown electric field and high saturation electron velocity have been attracting attention as the next generation of semiconductor materials for high-frequency/high-power devices. In particular, a multi-layer structure formed by laminating layers formed of AlGaN and GaN has the feature that high-concentration two-dimensional electron gas (2DEG) is generated at a lamination interface (hetero interface) owing to a large polarization effect (spontaneous polarization effect and piezo polarization effect) inherent in a nitride material, and hence a high electron mobility transistor (HEMT) using the multi-layer structure as a substrate has been developed vigorously (for example, see “Highly Reliable 250 W High Electron Mobility Transistor Power Amplifier”, Toshihide Kikkawa, Jpn. J. Appl. Phys. 44 (2005), p. 4896).
In order to put the above-mentioned HEMT device or a substrate for HEMT device that is a multi-layer structure used in manufacturing the same to practical use, various problems need to be solved: problems related to performance improvement such as increase of power density and efficiency, problems related to functional improvement such as achieving normally-off operation, and fundamental problems such as enhancing reliability and reducing cost. The above-mentioned problems are individually tackled vigorously.
For example, it is conceivable that controllable current density of a HEMT device, that is, power density capable of being utilized can be improved significantly if the concentration of two-dimensional electron gas existing in a substrate for HEMT device can be increased significantly. In a case of a nitride HEMT device having the most typical configuration in which a channel layer is formed of GaN and a barrier layer is formed of AlGaN, it is known that the concentration of two-dimensional electron gas increases along with an increase in AlN molar fraction of AlGaN that forms the barrier layer (for example, see “Gallium Nitride Based High Power Heterojunction Field Effect Transistors: Process Development and Present Status at USCB”, Stacia Keller, Yi-Feng Wu, Giacinta Parish, Naiqian Ziang, Jane J. Xu, Bernd P. Keller, Steven P. DenBans, and Umesh K. Mishra, IEEE Trans. Electron Devices 48 (2001), p. 552).
Further, growing attention is also paid to the HEMT device that has a low dependence on the piezo polarization effect, is capable of generating two-dimensional electron gas at high concentration almost only by spontaneous polarization, and has the structure with small strains, such as the HEMT device in which a channel layer is formed of GaN and a barrier layer is formed of InAlN (for example, see “Can InAlN/GaN be an alternative to high power/high temperature AlGaN/GaN devices?”, F. Medjdoub, J.-F. Carlin, M. Gonschorek, E. Feltin, M. A. Py, D. Ducatteau, C. Gaquiere, N. Grandjean, and E. Kohn, IEEE IEDM Tech. Digest in IEEE IEDM 2006, p. 673).
In a case of a HEMT device having an AlGaN/GaN heterostructure, when a barrier layer is formed of AlGaN having a large AlN molar fraction for increasing a two-dimensional electron gas concentration, a tensile stress generated inside the barrier layer increases. This leads to degradation in film quality and deterioration in surface form (such as increase of strains and occurrence of cracks). As a result, there occur various problems that “expected high two-dimensional electron gas concentration cannot be obtained (less than 2×1013/cm2 at most)”, “various contact characteristics such as schottky and ohmic ones become deteriorated”, “unnecessary surface level is formed, which deteriorates device dynamic characteristics”, and the like.
Further, growing attention is paid to a laminated structure capable of obtaining higher two-dimensional electron gas concentration almost only by spontaneous polarization, as the InAlN/GaN heterostructure disclosed in Medjdoub et al. above. As to such laminated structure, for example, it is proposed to form a channel layer of GaN, form a barrier layer of InxAl1-xN (x: to 0.18) having a composition that is lattice-matched to an a-axis of GaN, and further faun a thin spacer layer of AlN between the channel layer and the barrier layer. However, the means for achieving the above in a specific manner and other effective configuration example are not expressly described because there is a large difference in growth temperature between InN and AlN, which makes it difficult to control epitaxial growth in a mixed crystal composition containing both.